Bonding of rubber to fibrous reinforcers

ABSTRACT

Aromatic and aliphatic polyaminimides are disclosed as constituting a class of polyisocyanate precursors which can be effectively utilized in isocyanate-based adhesive systems for bonding rubber to fibrous polyester and like substrates.

United States Patent Inventors William J. McKillip Minneapolis; ClarenceN. Impola, Prior Lake, both of Minn. Appl. No. 881,879 Filed Dec. 3,1969 Patented Dec. 21, 1971 Assignee Ashland Oil, Inc.

Houston, Tex.

BONDING 0F RUBBER T0 FIBROUS RElNFORCERS 12 Claims, No Drawings u.s.c1"7/76 A, 117/122 P, 117/122 PF, 117/122 PA, 1171126013, 117/123 D,117/12s.4,117/132 B, 117/132 BE, 117/13s.s F, 117/145, 117/161 A,117/161 P,

117/161 ZA, 161/184, 161/185, 260/47 EN Primary ExaminerWilliam D.Martin Assistant Examiner-Bernard D. Pianalto AnomeysWilliam Kammerer,Larry W. Evans and Walter H. Schneider ABSTRACT: Aromatic and aliphaticpolyaminimides are disclosed as constituting a class of polyisocyanateprecursors which can be effectively utilized in isocyanate-basedadhesive systems for bonding rubber to fibrous polyester and likesubstrates.

BONDING OF RUBBER TO FIBROUS REINFORCERS BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to the treatment of afilamentary substrate to improve the bonding properties thereof whenutilized as the reinforcing element in the fabrication of vulcanizedrubber structures.

2. Description of the Prior Art In the manufacture of a variety ofvulcanized rubber goods reinforced with a fibrous textile material,particularly exemplary of which are pneumatic tires and powertransmission belts, the serviceability of the resultant structureslargely depends upon the strength of the adhesive bond formed betweenthe reinforcing element and the rubber in the course of thevulcanization step. The problem of adhesion is especially in theforefront in the manufacture of motor vehicle tires since the severeservice conditions to which these structures are subjected nowadays canresult in the development of extremely high temperatures and enormousdynamic stresses therein which in turn are prone to cause separation ofdelamination of the reinforcing element from the rubber.

In the initial phase of the tire industry, woven cotton fabrics werecustomarily employed as the reinforcing element. Mechanical keyingaround the protruding ends of the cotton staple and rubber strikethroughof the fabric that occurred upon vulcanization provided sufiicientbonding, thereby obviating the need of an adhesive. The subsequent useof rayon and the contemporary development of the bias ply tire, however,necessitated the use of a precoat of natural or SBR rubber latexadhesive to achieve a requisite degree of tie bonding. Later, with theintroduction of nylon as a reinforcer, it was found that the bondingproperties of the natural or SBR rubber latices were not suitable forthis purpose. This in turn led to the development of special elastomerlatices, e.g., a butadiene/stryrene/vinyl pyridine terpolymer and theuse thereof in combination with a phenol-formaldehyde condensate as theadhesive composition. Once the art settled on the preferred use ofresorcinol for preparing the latter, this adhesive system became knownas RFL.

With the advent of the polyester fiber era, the tire industry was soonto recognize the potential of this material as a rubber reinforcer. Whentested as such for tires, it is found that polyester provided a producthaving exceptional performance characteristics in terms of strength,durability and tread wear. Notwithstanding the overall excellentproperties of the polyester, the general use thereof in tireconstruction was delayed for lack of effective adhesive since the RFLsystem proved to be unsatisfactory for bonding rubber thereto.

The breakthrough in the development of an effectual adhesive compositionfor polyester came with the discovery of the polyisocyanate-basedsystem. Although many variations of the latter system have beenproposed, the one that has gained the most widespread acceptance is thetwo-dip method wherein the polyester is first treated with an aqueousdispersion of a polyepoxide and an aromatic polyisocyanate blocked withphenol, followed by treatment with a conventional RFL adhesive.

One of the principal disadvantages of the aforementioned two-dip methodstems from the fact that the blocked isocyanates are very difficult todisperse in water and consequently, an extensive ball milling procedureis required to obtain even at best a tolerably stable mechanicaldispersion of this component. Accordingly, uniform coating of thepolyester cord with such dispersions is tedious to maintain on theproduction line. An additional problem accompanying the use of theblocked isocyanate is that phenol is given off in the course ofresolving the complex. Phenol, besides being toxic,

also adversely affects the strength properties of polyester fiber.

SUMMARY OF THE INVENTION In accordance with the present invention, anovel adhesive coating composition for bonding rubber to a filamentarysubstrate of glass or a synthetic polymer is provided which comprisesthe heat reaction product of a polyepoxide and a latent polyisocyanatein the form of a compound having a plurality of tertiary aminimideradicals. Such coatings are obtained by applying to the substrate anaqueous dispersion of a polyepoxide containing a polyaminimide dissolvedor dispersed therein and thereupon heating to effect concomitant dryingand rearrangement of the polyaminimide to yield the correspondingpolyisocyanate for reaction with the polyepoxide component.

The foremost advantageous feature of the polyaminimides of thisinvention in attaining the objectives thereof is that most of suchcompounds are completely water soluble. In these few instances where notcompletely water soluble, the compounds are nonetheless highlyhydrophilic and therefore can be readily dispersed or emulsified in anaqueous medium. Consequently, in the application of the aqueous emulsionof the polyaminimide-polyepoxide combination, excellent wetout of thesubstrate readily occurs and thus no problem is encountered inconsistently obtaining coatings of uniform composi' tion.

The polyaminimides useful herein can be rearranged to the correspondingpolyisocyanate at temperatures ordinarily observed in unblocking thecomplexed isocyanates of the prior art. Such rearrangement, however,results in a tertiary amine leaving group having essentially nodeleterious effect on polyester. Accordingly, the toxicity anddegradation problems attendant in the use of the conventionalphenol-blocked isocyanates are obviated in the practice of thisinvention.

Still another potential advantage of the present invention is attributedto certain inherent characteristics of the polyminimide as isocyanateprecursors. Since the thermolytic conversion of the polyaminimide is atime-temperature dependent chemical reaction, the rate of the resultantrearrangement can be readily controlled decidedly better control thanthe rate experienced in resolving an isocyanate-phenol complex.Consequently, the rearrangement characteristics of a polyaminimide canbe expected to provide a polyurethane complex with the polyepoxidehaving free aminimide radicals capable of further reacting with the RFLcoating or for that matter with any free active hydrogen groups of thesubsequently applied rubber as well as such groups contained bycompounds that might be incorporated therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the reaction productresulting in the practice of this invention in itself serves as anadhesive for bonding a vulcanizable rubber to the substrate, thepreferred embodiments of the invention contemplate that the substratealso bears a coating of conventional RFL adhesive. The latter can beapplied after or before treating the substrate withpolyepoxidepolyaminimide reaction product. Further details concerningthe applicable RFL adhesive coating compositions will be set forthhereinbelow.

The polyaminimides from which the novel adhesive coatings of thisinvention can be obtained have the following general formula:

wherein A represents an aromatic, aliphatic or cycloaliphatic residue, Ris alkyl or hydroxyalkyl and n is 2, 3 or 4.

Illustrative of the mechanism whereby the aminimide radical rearrangesto an isocyanate group is shown in the following schematic.

An extensive, although not exclusive enumeration of polyaminimidesuseful in accordance with this invention together with a description andexemplification of a method for the preparation thereof are set forth inU.S. Pat. No. 3,450,673. Briefly, such method consists of reacting apolycar boxylic acid chloride with a unsymmetrical disubstitutedhydrazine followed by quaternizing the resultant acid hydrazide andfinally treating with a strong base to yield the aminimide. In light ofthe fact that the polyaminimide is to be converted to the correspondingpolyisocyanate in carrying out the present invention, it is desirable togenerate a leaving group as volatile as possible. Accordingly, it ispreferred to employ a hydrazine appropriately substituted with loweralkyl groups in the preparation thereof. Likewise, the use of a lowmolecular weight quatemizing agent, as for example, the C,-C., allzylchlorides, is indicated.

An alternate and more efficient manner for preparing the polyaminimidesstructurally noted above is described in U.S. Ser. No. 703,554, filedFeb. 7, 1968, now U.S. Pat. No. 3,485,806. This method consists ofreacting a carboxylic acid ester, preferably those esters wherein theester group is a lower alkyl, with an equivalent of each of anunsymmetrical di-(lower)alkyl hydrazine and a lower alkylene oxide. Afurther preparatory route involving the use of a polycarboxylic esterconsists of reacting the ester preferably in an aprotic solvent, withabout a stoichiometric amount of tri-(lower)allcyl hydrazinium halide inthe presence of approximately an equivalent proportion of a strong base.The latter method is described in detail in application Ser. No.514,705, filed Dec. 17, 1965 and now abandoned.

A variety of polyepoxides are applicable for coreacting with thepolyisocyanates in accordance with this invention. A preferred class ofepoxides are the dehydrohalogenation condensation products of a polyoland at least two equivalent portions of epichlorohydrin. A particularlypreferred polyol for this purpose is glycerine as the resultant epoxideproducts are easily handled liquid materials having a high oxiraneoxygen content, e.g., epoxide equivalents ranging from about 100 to 200.Still another suitable type of polyepoxide is that derived by similarlycondensing epichlorohydrin with an aromatic polyol, specificallybisphenol. Preferred products of this type for the purposes hereinexhibit a molecular weight not in excess of about 1,500 and an epoxideequivalent ranging from about 200 to 1,000. While the latterpolyepoxides are normally solid materials, they can nevertheless bereadily dispersed in an aqueous medium.

As indicated hereinabove, the reaction mixture comprised of thepolyaminimide and the polyepoxide is applied to the substrate in theform of an aqueous emulsion of the polyepoxide containing thepolyaminimide dissolved therein. 1n formu lating such emulsions, theratio of the indicated reactants can range from about 90 to parts of thepolyaminimide and correspondingly from about 10 to 90 parts of thepolyepoxide. A preferred range of said reactant mixtures is from about60 to 40 parts of the polyaminimide and correspondingly from 40 to 60parts of the polyepoxide. The total solids content of the emulsion isordinarily adjusted so that from about 0.2 to 5 percent by weight ofsaid solids based on the substrate is deposited in a one-dipapplication. Other adjuvants such as viscosity control agents,surfactants, latices of elastomers having a high hydroxyl value and thelike can be used if desired. Other means of applying the dispersion tothe substrate can be observed, but the clipping process has beenuniversally accepted by the tire industry as representing the mostconvenient manner of application. After treating the substrate in theforegoing manner, it is then heated at a temperature in excess of 150 C.and preferably at least about 200 C. for a time ranging from about 1minutes.

In the preferred embodiments of this invention, the substrate also bearsa coating of a conventional RF L adhesive and as previously mentioned,the latter can be applied after or before treating the substrate withthe polyepoxide-polyaminimide reaction product. Normally, athermosetting-type resorcinol-aldehyde condensate is utilized in an RF Ladhesive. The

preferred condensates of this type are obtained by condensing resorcinolwith formaldehyde or a compound generating formaldehyde in a molar ratioof from about 110.9 -l:2, respectively, employing a basic catalystcapable of providing a pH in the range of from about 8 -1 1. 1n thepractice of the present invention, however, it has been noted that theresorcinol-formaldelhyde condensates can acceptably be of the fusible ornovolak type. These are conventionally prepared by observingresorcinol-formaldehyde ratio of less than about 1.0, typically in therange offrom about 0.5 to 0.7, and carrying out the condensation in thepresence of an acid catalyst.

The rubber most satisfactorily adapted for preparing the RFL adhesivecomposition is a butadiene-styrene-vinylpyridine terpolymer of which anumber of products of this type are commercially available. Usually, 10to 20 parts of the phenolic resin and correspondingly from to parts oflatex solids are combined in formulating the adhesive. Total solidcontents of the RF L composition will depend upon the particular type ofmaterial used as the reinforcer, but generally ranges from about 10 to25 percent.

Although the salient objective herein is to provide improved adhesivesfor bonding rubber to polyester, the usefulness thereof is obviously notlimited to such an application. The novel adhesives of this inventioncan likewise be advantageously utilized in obtaining reinforced rubberstructures wherein the fibrous reinforcer exhibits a similarrecalcitrancy toward bonding with rubber as does polyester. Notableexamples of such reinforcers are hydrophobic materials such asfiberglass, polyolefm fibers, steel wire cord and the like. Moreover,these adhesives can be used to bond rubber to rayon and nylon to obtainreinforced structures exhibiting strength properties substantiallybeyond that provided by the conventional RF L adhesives. Also theadhesives of this invention can be employed to bond rubber to plys ofdissimilar substrates as in the construction of the belted tires whereinfiberglass fabric belts are laid over conventional plys of polyester,nylon or rayon.

The following examples of specific embodiments are presented toillustrate the best mode contemplated for carrying out this invention.As indicated, these examples are given by way of illustration and,accordingly, any enumeration of detail set forth therein is not to beconstrued as limiting the invention except as such limitations appear inthe appended claims. All parts are parts by weight unless otherwisespecified.

EXAMPLE I This example serves primarily to illustrate the effectivenessof a polyaminimidepolyepoxide adhesive for bonding polyester cord torubber in a one-dip operation. The polyester cord of this example was athree-ply filament of the type conventionally used in the manufacture ofmotor vehicle tires. The method of coating consisted of passing the cordthrough the respective adhesive emulsions employed in this example andthereupon drying at 200 C. for the times indicated in table 1 under theheading Thermolysis." The treated cord's adhesion to rubber wasdetermined in accordance with the standard H" test (ASTM Method No.D2138 62T). A vulcanizable rubber compound of the following recipe wasemployed in this test.

Parts Natural rubber No. l smoked sheet 35 cisl, 4-polybutadiene rubber20.0 SBR rubber 67.5

FEF carbon black 55.0

Zinc oxide 10.0

Stearic acid 10 Pine tar oil 7.5

Antioxidant BLE (Naugatuck Chem. Co.) 0.5 Sulfur 3.0

Resin (Resorcinol/HCHO-l/ 0.7 moles, resp. 0.75 CBS(Ncyclohexyl2-bcr\zothia1ole) 0.8 DPG (diphenylguanidine) 0.2

TABLE I Poly- Dip composition (parts) Ave. amin- Thermol- H imide PolWater ysis, min. value, Run No. (PAI) RF PAI PE BD3 (total) at 200 C.lbs.

1 (control)- 1.8 2 A 4 4 72 2 12.8 3. A 4 4 10 82 2 9.0 4. A 4 4 10 82 413.7 5 B 4 4 10 82 2 9.4

l Resorcinol (1.0 M)/HCHO (0.6 M) acid catalyzed condensate. 9Polyepoxide (Epon 812, Shell Chem. Co. 3 Hydroxyterminated polybutadiene(Poly BD 11-46 M, Sinclair). A =Trimethylamine trimellitylirnide.B=Trimethylarnine terephthalylimide.

EXAMPLE 11 a le lll-ienl lle L This example illustrates the use ofpolyaminimides in ac- 233 3;? Ave cordance with this invention for atire cord adhesive applica- R 1st dip composition (parts) S lt1O1n *1 Rb 1 11H 0 yam 11- u V8. 110, tron following the procedure conventionallyobserved in a RF HCHO Latex 2 water 3 M) her lbs. two-dip operation. Thetest cord was the same as that em- 4 2 16 78 0* II 22.1 ployed inexample 1. Drying of the dipped cord in each in-. 2 1 8 89 H 10.5 stancewas effected at 220 C. for 2 minutes. The adhesive 2 g g g g3 values(ASTM D3138 -62T) noted for the various test runs l CR8512resorcinol/HCHO fusible resin (Ashland Chem. 00.). together withcompositions of the respective dips are outlined 2 Gamma vinyl pyridinelatex (The Gen er a1 Tire and Rubber nll elollgwlngtablell; MW Wm W iCf. Table II supra.

We claim: in 1. A rubber-reinforcing element in the form of afilamentary TABLE H substrate of steel, glass, rayon or a syntheticorganic polymer having a coating comprising the in situ reaction productof 1st dip compo. Avg from 90 to 10 parts by weight of l) a polyepoxidehaving an 211d p composition epoxide equivalent weight of from about 100-l 000 and an Run (polyamin- Rubvalue, No imidey A1) HCHO Latex 2 waterher average molecular weight not in excess of about 1,500; andcorrespondingly from about 10 to 90 parts by weight of (2) a 1 C 4 2 1678 I 23.8 2 C 2 1 16 81 I 23 2 compound having the formula: 3 D 4 2 1678 II 28. 3 0 a 1 s as H M l 0. 2 s E 2 1 a 89 II 29.7 AL(3 N*N(R 3 7 E4 2 16 78 II 33.6 g E; 2 g 12 g: h 352% wherein A represents anaromatic, aliphatic or cycloaliphatic 10- H 4 2 16 7 II 30. 9 residue, Ris alkyl or hydroxyalkyl and n is 2 4.

e. E bis-((1 .1-dimethyl-1- (2-hydroxypropyl) amine) suberylimide.

idF =1,4-bis-((1,l-dimethyl-l-(2-hydroxypropyl)amine))cyclohexanoylirn-G=bis-((1,l-dimethyI-l-(2-hydroxypropyl;amine);adipylimide.H=bis-((1,l-dimethyl-l-(2-hydroxypropyl amine) aze aylimide. I= Rubbercompound according to recipe given in Example I. II P rgpriet tire treadrecipe (Uniroya EXAMPLE III in this example adhesive values weredetermined for a polyester tire cord treated with selected adhesivesystems of example 11 but wherein the conventional method of firstfonning a polyurethane coating thereon followed by overcoating with theRFL was reversed. Drying conditions employed in these test runs were thesame as in example II. The results mad are waetaee p y nsa 1.

TABLE III 2nd dip compo- Ave. 1st dip composition (parts) sition "H Ru(polyamin- Rubvalue, No. RF 1 HCHO Latex 1 Water imide-PAI) her 1 s.

l.. 4 2 16 78 O I 25.4 2 2 1 16 81 C I 21. 9

2. A rubber-reinforcing element in accordance with claim 1 wherein saidcoating comprises the in situ reaction product of from 60 to 40 parts byweight of said polyepoxide and correspondingly from 40 to 60 parts ofsaid compound.

3. A rubber-reinforcing element in accordance with claim 2 wherein thesubstrate has from about 0.2 to 5 percent by weight of said coating.

4. A reinforced rubber structure wherein the reinforcing element is acoated substrate in accordance with claim 3.

5. A reinforced rubber structure in accordance with claim 4 wherein saidsubstrate is a polyester cord.

6. A reinforced rubber structure-in accordance with claim 5 wherein saidpolyester cord additionally bears an adhesive coating of a latex and aresorcinol-aldehyde condensate.

7. A reinforced rubber structure in accordance with claim 5 wherein A ofthe formula of said compound represents a terephthalylene radical.

8. A reinforced rubber structure in accordance with claim 5 wherein A ofthe formula of said compound represents an adipylene radical.

9. A reinforced rubber structure in accordance with claim 5 wherein A ofthe formula of said compound represents an azelaylene radical.

10. A reinforced rubber structure in accordance with claim 5 wherein Aof the formula of said compound represents a dithiodipropionyleneradical.

1 l. A reinforced rubber structure in accordance with claim 5 wherein Aof the formula of said compound represents a suberylene radical.

12. A reinforced rubber structure in accordance with claim 5 wherein Aof the formula of said compound represents a cyclohexanoylene radical.

2. A rubber-reinforcing element in accordance with claim 1 wherein said coating comprises the in situ reaction product of from 60 to 40 parts by weight of said polyepoxide and correspondingly from 40 to 60 parts of said compound.
 3. A rubber-reinforcing element in accordance with claim 2 wherein the substrate has from about 0.2 to 5 percent by weight of said coating.
 4. A reinforced rubber structure wherein the reinforcing element is a coated substrate in accordance with claim
 3. 5. A reinforced rubber structure in accordance with claim 4 wherein said substrate is a polyester cord.
 6. A reinforced rubber structure in accordance with claim 5 wherein said polyester cord additionally bears an adhesive coating of a latex and a resorcinol-aldehyde condensate.
 7. A rEinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents a terephthalylene radical.
 8. A reinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents an adipylene radical.
 9. A reinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents an azelaylene radical.
 10. A reinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents a dithiodipropionylene radical.
 11. A reinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents a suberylene radical.
 12. A reinforced rubber structure in accordance with claim 5 wherein A of the formula of said compound represents a cyclohexanoylene radical. 